I think you probably want it build it a little higher, I'm not sure it would be worth your time and materials for a 1 foot gain in height.

Also, Plywood is typically sold in 4X8 ft. sheets. So if you used 2 sheets you would have an 8X8 ft stage or 3 sheets would yeild an a 12X8 ft. stage and so on.

I would first frame out a box using 2x4's for the stage, adding cross supports as neccessary. (everything would be connected using heavy duty nuts and bolts, except for the plywood which I would attach with screws).

Once the frame and support was constructed I would use 4X4's for legs (say 3 feet tall or so) I would place one leg in each corner and one in the center of the stage at the cross supports and maybe one at the edge/center of each plywood sheet.

Once the legs were attached I would add 2 cross member supports (2X4) at a 45 degree angle attached to each leg connecting to the 2X4 box.

I would then see how sturdy the thing was and maybe add another 2X4 at the center of each leg creating another box.

I would then label all of the parts and take some pictures of how it looks together, dissamble it, transport it to the playa, reassamble, then at the end of the week remove the hardware and burn.

If you notch out the support beams where they overlap, you can "Lincoln Log" the frame together with very little (or NO!) hardware, making it unnecessary to pack tools and de-metallize prior to burning.....

My theatre friends tell me that a stage doesn't have to be a rectangle - it can be T shaped, or V shaped, etc. to give you traveling room, but also save on materials.

That said, I built a small 4X8 stage last year, that doubled as a carrying cart (just flip it over and add a couple of wheel assemblies with a few screws.) The walls of the cart became supports for the stage platform (when fliped that way) keeping it from tweaking side-to-side or front to back. They were 24" high, 1/2 inch OSB held together with 2x4's in the corners and in the centers of the long sides (all put together with wood screws). To support the actual stage platform (the bottom of the cart), I ran 8 foot 2X4's across both long sides, supported by the other 2X4's and placed them 3 1/2 inches from the stage platform. Before flipping it over, I can add as many 2X4 cross supports as I need by just slipping them into the gap between the platform and the 8 foot 2X4's. they need no screws - friction and gravity keep them right where they belong.

This all broke down into flat sections for transport and went together in about 15 minutes with a cordless drill. I did not burn it!

I have built several sets of risers for both stages and audiences for my theaters. A plywood top on 2x4 sides is the standard all-wood framing technique (although 1x4 can be used to save weight but it is more expensive even for less wood because 2x4 is the typical standard framing wood and is readily available at Home Depot for about $2 an 8' stick - the most important thing is the 4 inch height, not the 2 inch width of a 2x4 and a 1x4 is almost as stable on its 4 inch edge as a 2x4 is).

I typically make these in 4'x4' sections (1/2 sheet of plywood) so they are easier to move/carry by myself. I also make a few 2'x4' sections and I have even made a few 4'x4' half triangle sections so I can put up weird shape stages/platforms.

We have always used a really very simple and elegant method for riser legs that work up to a height of about 36" and are extremely stable. Each riser leg is made out of two pieces of plywood that are notched so that they fit together like those pieces of cardboard you find as dividers in wine cartons. At the top of each piece the notch is much wider to a depth of about 3-1/2" (almost the same as the height of a piece of 2x4). When assembled, the notch at the top carries two pieces of adjacent 2x4 cleanly (or the corner of four platforms). With 4x4 platforms, we use one pylon for each corner and it is amazingly stable. There is no need to screw the platform pieces together (the pylons hold everything totally solid). If the height exceeds 24", I will occasionally run cross bracing between the pylons, but this isn't really necessary. Here is a typed view of the pylons:

A couple of things to notice are that the long narrow slots (which are used to mate the two pieces, one running down from the top and the other running up from the bottom) are NOT exactly half the height of the piece. In fact they can (and should) be somewhat longer than half the height. When placed on smooth ground, the pylon is more or less self leveling. The long narrow slots need to be pretty close to the thickness of the plywood you use so that the pylon pieces fit together snugly (but not too snug, you have to be able to take them apart too). If you use 3/4" plywood, I would make the narrow slots about 13/16". The top wide slots (call them the "crotch") have to be just over 5.25" wide. 5" is the hypotenuse of a square 3.5" x 3.5" (the square formed by two piece of 2x4 on edge placed side by side. In order to allow for imperfections in matching the platforms together, I cut the crotch slots slightly large.

The pylon height can be anything you want, but the important dimensions are the shoulder of the crotch slots (these MUST be just under 3.5" so they carry the full depth of the 2x4 but dont push up on the plywood top).

You can cut these pylons most easily on a bandsaw (we did them four at a time in a sandwich). The narrow notches can be cut so that at the bottom you just angle in leave a sort of V (just make sure that the notch is absolutely STRAIGHT at least 1/2 of the height of the pylon). This saves time in trying to cut right angles in a notch. The shoulder notches must be flat to carry the 2x4s.

I hope this makes sense. This is a completely NO hardware way to put legs on platforms, only wood (not even screws are needed in the pylons!)

I have built several sets of risers for both stages and audiences for my theaters. A plywood top on 2x4 sides is the standard all-wood framing technique (although 1x4 can be used to save weight but it is more expensive even for less wood because 2x4 is the typical standard framing wood and is readily available at Home Depot for about $2 an 8' stick - the most important thing is the 4 inch height, not the 2 inch width of a 2x4 and a 1x4 is almost as stable on its 4 inch edge as a 2x4 is).

I typically make these in 4'x4' sections (1/2 sheet of plywood) so they are easier to move/carry by myself. I also make a few 2'x4' sections and I have even made a few 4'x4' half triangle sections so I can put up weird shape stages/platforms.

We have always used a really very simple and elegant method for riser legs that work up to a height of about 36" and are extremely stable. Each riser leg is made out of two pieces of plywood that are notched so that they fit together like those pieces of cardboard you find as dividers in wine cartons. At the top of each piece the notch is much wider to a depth of about 3-1/2" (almost the same as the height of a piece of 2x4). When assembled, the notch at the top carries two pieces of adjacent 2x4 cleanly (or the corner of four platforms). With 4x4 platforms, we use one pylon for each corner and it is amazingly stable. There is no need to screw the platform pieces together (the pylons hold everything totally solid). If the height exceeds 24", I will occasionally run cross bracing between the pylons, but this isn't really necessary. Here is a typed view of the pylons:

A couple of things to notice are that the long narrow slots (which are used to mate the two pieces, one running down from the top and the other running up from the bottom) are NOT exactly half the height of the piece. In fact they can (and should) be somewhat longer than half the height. When placed on smooth ground, the pylon is more or less self leveling. The long narrow slots need to be pretty close to the thickness of the plywood you use so that the pylon pieces fit together snugly (but not too snug, you have to be able to take them apart too). If you use 3/4" plywood, I would make the narrow slots about 13/16". The top wide slots (call them the "crotch") have to be just over 5.25" wide. 5" is the hypotenuse of a square 3.5" x 3.5" (the square formed by two piece of 2x4 on edge placed side by side. In order to allow for imperfections in matching the platforms together, I cut the crotch slots slightly large.

The pylon height can be anything you want, but the important dimensions are the shoulder of the crotch slots (these MUST be just under 3.5" so they carry the full depth of the 2x4 but dont push up on the plywood top).

You can cut these pylons most easily on a bandsaw (we did them four at a time in a sandwich). The narrow notches can be cut so that at the bottom you just angle in leave a sort of V (just make sure that the notch is absolutely STRAIGHT at least 1/2 of the height of the pylon). This saves time in trying to cut right angles in a notch. The shoulder notches must be flat to carry the 2x4s.

I hope this makes sense. This is a completely NO hardware way to put legs on platforms, only wood (not even screws are needed in the pylons!)